Last year, 11-year-old Colby Wentlandt sketched out his running goals in an online blog called “A Mile with Colby.” They included “a marathon when I am 12, a 50-miler when I’m 13 and a 100-miler when I’m 14.”

Less than a year later, Colby, the son of Marine Master Sgt. Brady Wentlandt and Shawna Wentlandt, has checked the first two items off his to-do list and is closing in on the third.

Colby is an ultrarunner, a member of a unique club that eschews the conventional 26.2-mile marathon distance in favor of longer, more grueling runs. On Jan. 1, he completed 56 miles of a 100-plus-mile race in Glendale, Ariz.

Two months later, he ran 100K — 62 miles — at another race.

“On reason I like running is you can’t blame anyone else for having it go wrong. It’s all you, the blame and the accomplishment,” Colby said during a recent telephone interview from his Warner Springs, Calif., home.

The child of a dedicated mom and dad who took up marathons, and later ultras, in 2011, Colby began taking part in “fun runs” and 5Ks at age 8. He moved on to longer races a few years later after joining his elementary school running club.

His presence at 10Ks and half-marathons often draws admirers. But now that he’s striking out on distances most accomplished marathoners won’t attempt, his participation also is attracting some critics.

“We see the comments on race pages. At one ultra, I overheard a woman make a backhanded remark: ‘So, we can’t spank our kids anymore but we can make him run a 100K?’” Shawna Wentlandt said.

The concern is whether the endurance sport will impede his physical development or cause lifelong musculoskeletal problems.

The medical community seems to be divided.

“If the current trend continues, in 30 years we’ll have a crop of adults with serious chronic injuries that require surgery and aggressive treatment,” said Dr. Amy Valasek, a pediatric sports medicine expert at Johns Hopkins University in Baltimore who sees an average 100 children per month with sports injuries.

But “There’s no data to show that it’s healthy or harmful,” said Dr. William Roberts, medical director of the Twin Cities Marathon and professor of family medicine at the University of Minnesota. “We’re starting to look at the data from kids who ran marathons in the 1980s and are finding that many of them are still running today.”

The intensity of training and long-distance races on young athletes can make them vulnerable to overuse and growth-plate injuries as well as damage to the growing tissue at the ends of their long bones, according to the National Institutes of Health.

Boys are more vulnerable than girls to growth-plate damage, and both genders are susceptible to overuse injuries, the result of chronic strain or a poorly healed acute injury.

Contributing to the growing problem of sports-related injuries among youngsters are the facts that children now specialize in one sport at younger ages and are engaged in rigorous training regimens, Valasek said.

“The combination of repetitive use and skeletal immaturity puts these youngsters at high risk for injuries, some of them long-lasting, so it is really important that young children have whole-body conditioning and engage in a variety of athletic activities rather than one sport,” Valasek said.

More than physical strain

And it’s not just the physical aspect of endurance sports that challenges young athletes. There’s the mental strain of competition and training that many children aren’t mature enough to handle, sports psychologists say.

Mark Hyman, a journalist who interviewed hundreds of parents, physicians, children and psychologists for his book “Until it Hurts: America’s Obsession with Youth Sports,” said the mental pressure to succeed often causes youngsters to burn out long before they meet physical roadblocks in their sport.

“We look at children like they are mini adults. They aren’t. They have physical and mental limitations that are much different,” Hyman said. “I’m not suggesting sports are a negative in any way. Many kids are focused and motivated. But parenting is about enforcing balance.”

Marine Lt. Col. Stephen Armes, an accomplished triathlete and member of the All-Marine Triathlon Team, has two sons, Erik, 16, and Ryan, 14. Both have run since they were 6 years old and competed in triathlons since age 8. They are members of their high school running squads and also compete in sprint- and standard-distance triathlons.

When Erik developed a stress fracture last year after nearly 10 years of injury-free sports, his coaches and parents made sure he dialed back on competitions and training in order to heal. They also continue to watch him carefully.

The hardest part of being a parent “is trying to ensure they keep a balance, and regardless of any success they may have, that they stay grounded. No matter how fast you are, there is always someone out there faster, so just focus on doing the best you can and have fun,” Armes said.

Guarding against breakdown

Colby Wentlandt is a child known to push himself. After completing 50 miles of his 56-mile race in less than 12 hours, he continued for six more miles despite extremely sore muscles and exhaustion. His parents pulled him from the course despite his protests.

But he also needs instruction and coaching, because like most children, he prefers the excitement of races and crispy chicken nuggets to training and healthier fare, according to Shawna.

“I did a lot of research when Colby started telling us he wanted to do this. As long as you watch these kids and monitor them, you can let them go. Our bodies are machines. They don’t break down if you are careful and smart,” Brady Wentlandt added.

Valasek recommends youngsters play a variety of sports and participate in no more than five days per week of sport-specific training.

Roberts said parents should support their children’s athletic endeavors but also make sure the rest of their lives are balanced.

“First, I’d ask, whose idea is this? If it’s coming from the child, OK. Then, parents need to be asking and watching. Are they doing well physiologically, psychologically, academically and with social growth and spiritual growth? Are they injury-free and enjoying it? Let them do it,” he said.

The week of March 28, Colby ran more than 100 miles in two separate events. He says he does it because he loves seeing friends along the trail, running with his parents and “getting the belt buckles” that runners earn.

He’s added a new goal to his list — to become the youngest finisher at Badwater, the 135-mile race in Death Valley, Calif..

“Why do I want to run it? First of all, the title seems like a challenge. And second, it’s the coolest race in the world,” Colby said. ■